The present invention relates to a carrier for a developer and a method of electrophotographically producing a visual toner image by using the developer containing such a carrier. More particularly, the present invention relates to an improved carrier serving as a constituent of a developer and having particular shape and size, and a method of electrophotographically forming a visual toner image using the developer containing such a carrier, thereby obtaining a toner image with a high density and a high quality and achieving a compactness of apparatus.
In conventional electrophotographic recording apparatus such as printers, facsimiles, etc., it is known that a visual toner image is produced by the successive steps of (1) forming an electrostatic latent image corresponding to original image or information data, for instance, on a cylindrical image-bearing member, (2) magnetically attracting a magnetic developer on a rotatable developing roll equipped with an inner permanent magnet and disposed opposing the image-bearing member, (3) delivering the magnetic developer, while forming a magnetic brush, to a developing zone where the electrostatic latent image on the image-bearing member is slidingly brushed with the magnetic brush. The developed image is then transferred to a recording sheet and fixed thereon by heating.
After transferring the toner image to the recording sheet, a small amount of the toner is likely to remain on a photosensitive drum as an image-bearing member in such an electrophotographic recording apparatus. Thus, a cleaning device is generally provided to remove the residual toner from the image-bearing member. To this end, a space for installing the cleaning device must be provided in the vicinity of the image-bearing member, failing to achieve an intended compactness of the electrophotographic recording apparatus.
To accomplish the compactness of an electrophotographic recording apparatus, it has been proposed that the cleaning device be replaced by a so-called developing-cleaning unit having developing and cleaning functions to remove a residual toner from the image-bearing member (Japanese Patent Laid-Open No. 4-86878). In the electrophotographic recording apparatus equipped with such a developing-cleaning unit, a magnetic developer containing a toner and a spherical magnetic carrier is employed. However, in the case of using such a magnetic carrier of a spherical shape, a relatively small specific surface area of the magnetic carrier leads to a small contact area of the carrier and the toner, resulting in a small triboelectric charge of the toner and a low image density, thereby failing to obtain a clear toner image.
Further, proposals have been made to form a fine toner image by reducing a size of the magnetic carrier. By using the magnetic carrier having a reduced size, a toner image with a high resolution and a high quality can be obtained due to the formation of a thin developer layer. However, it is difficult to retain the magnetic carrier with a reduced size be well magnetically on the developing means, because the magnetic carrier is likely to scatter, thereby causing problems such as the contamination of the developing means and nearby elements, deterioration in a quality of the toner image, etc.
Furthermore, when one developing cycle is performed by one revolution of the image-bearing member in the electrophotographic recording apparatus equipped with the developing-cleaning unit having the developing and recovering functions, the toner remaining on the image-bearing member after a transferring step cannot completely be removed by the developing-cleaning unit, so that the residual toner will attach to the image-bearing member in an image-forming region. If the residual toner is not completely recovered, the resultant toner image on the recording sheet suffers from poor quality. To eliminate such a problem, there has been proposed a system in which one developing cycle is performed by two revolutions of the image-bearing member, achieving the complete recovery of the residual toner. However, when such a system is employed, an image-forming rate is low, failing to meet a recent demand for a rapid visualization of information.